Measuring systems of the generic type are typically used for controlling printing processes. In this application, measurements are usually taken of color measuring bars of the various printing inks used for printing. The color measuring bars typically have a side length of a few millimeters and are usually printed alongside the edge of the actual printed image.
These days, standard print control bars used in a standard printing process are measured either with conventional handheld spectral photometers or with scanning measurement systems such as the Intellitrax System made by the X-Rite company or the AxisControl System made by Heidelberger Druckmaschinen, for example. These systems are equipped with a single spectral measuring head. The Intellitrax System also has a track finding system for automatically guiding and positioning the measuring head along the print control bar and synchronizing the measuring rate.
The printed surface occupied by the simultaneously printed color measuring bars leads to paper wastage and there is therefore a definite need to keep this surface as small as possible for reasons of cost.
Using smaller color measuring bars increases measuring requirements from various points of view.
Smaller measuring bars are difficult to measure using handheld devices. Below a certain size, it becomes practically impossible to position the measuring device precisely by hand. For very small color measuring bars, therefore, it is necessary to use an automatic measuring system with electronically assisted positioning technology in order to determine the exact position of the color measuring bar on the measurement object.
The size of the color measuring bar is also limited by the uniformity of the printing process and the scanning frequency used. The measuring technology must be capable of determining a representative mean value, which is independent of local fluctuations in the way the ink is applied or of the relative positioning of the scanning structure. Too small color measuring bars do not enable a mean value to be determined satisfactorily. One way of getting round this problem is to sub-divide the color measuring bar into a plurality of smaller color measuring elements. This enables these color measuring elements to be distributed at points across the entire measurement object where they will not cause interference. Measuring a lot of smaller color measuring elements enables finer measurement values to be obtained (from the same color measuring elements and color measuring elements with the same spectral reflection properties), thereby resulting in more representative measurement values.
Measuring speed or measuring time is also an important factor in a productive printing process. Ideally, it should be possible to measure a lot of these small color measuring elements at the same time (in parallel), for which purpose image detection and image processing techniques are necessary. One problem of image processing technology in connection with relatively small color measuring elements is the considerable amount of optical crosstalk which occurs, caused by the lighting of the image areas adjacent to the color measuring elements. Typical crosstalk values are in the order of 1% or higher, which is not acceptable for process control and quality assessment applications. It is therefore necessary to take steps to eliminate crosstalk.
A measurement field layout with smaller measuring fields was introduced by FOGRA and UGRA under the name of Minitarget. Such a Minitarget comprises 12 color measuring bars which are arranged in a fixed, predefined pattern. However, the overall dimensions of the Minitarget are still 10 mm*13 MM, which means that paper wastage can not be totally avoided. The Techkon company has developed a camera-based density measuring system (MTC920) which is suitable for measuring and analyzing such Minitargets.
Documents EP1213568B1 and EP1213569B1 describe spectral image measuring techniques with digital crosstalk compensation. However, these techniques require complicated characterizations and are only suitable for small measurement objects to a limited degree.